Use is generally made, in black-and-white developing solutions, of an organic developing agent chosen from di- and poly- hydroxybenzenes and reductones. The most commonly used reductones are described in U.S. Pat. No. 2,691,589, in particular ascorbic acid, its steroisomers, diasteroisomers and derivatives of the carbohydrate type.
Metallic ions such as Fe.sup.2+, Ti.sup.3+, V.sup.2+, Cr.sup.2+ are also capable of reducing silver ions into metallic silver, and developing solutions comprising organo-metallic complexes have been known for a long time. Thus the French patent BF 1,068,805 describes a development process using organo-metallic complexes of iron or titanium and aminopolycarboxylic acids such as ethylenediaminetetraacetic acid. Other developers comprising complexed metals are described in The Theory of the Photographic Process, T. H. James, Ch 11, 4th Edition, pages 294-298 and in Photographic Chemistry and Physics, Glafkides, 5th Edition, Chapter VI, pages 121-123.
The developers comprising organo-metallic complexes have advantages since they easily dissolve-in water, are active in an extensive pH range, are not required to be used in a highly alkaline environment and form a completely reversible oxydo-reduction system. They can be regenerated by electrolysis, as described in U.S. Pat. No. 5,310,631, by contact with steel wool, as described in U.S. Pat. No. 3,945,828, or by ultraviolet irradiation as described by Y. Shirai, in Papers from International Congress of Photographic Science, 1982, pp 312-314, Photographic Abstracts ed. The possibility of regenerating this type of developer makes it possible to obtain ecological developers by minimizing the volume of effluent, which helps to avoid water pollution. However, they have not been given a favorable reception in photography since they act slowly and give low-contrast images.
Attempts have been made to reduce the development time by combining an organic developing agent with the organo-metallic complex. For example, Research Disclosure, Article 15034, Vol 150 of October 1976, describes a developing composition comprising a metal complexed with a polycarboxylic acid and a pyrazolidone or a p-phenylenediamine.
FR 2,241,810 describes the association of an iron chelate, ascorbic acid and a codeveloper, which may be a phenidone. These compositions are alleged to be stable in air, capable of rapid development and have the possibility of being partially regeneratable.
Known phenidones have low solubility in water, which presents drawbacks with regard to the manufacture of the developer and its ease of use.